In the process of exploration and development of hydrocarbons, wells are drilled using drilling fluids. These drilling fluids are composed of liquids that are weighted with fine grained solids like barite which increases the density of the drilling fluids to exceed the pressure of the fluids in the formation rock pores. This keeps the formation fluid in place while drilling and prevents the formations fluids from being produced to the surface in an uncontrollable manner which is commonly know as a blowout. Since the pressure of the mud system exceeds the formation pore pressure the mud fluids (know as filtrate) will flow into the formation. This process is called invasion. The mud systems are designed to minimize this invasion by forming a mud cake composed of the solids being deposited on the well bore walls. It is desirable to obtain formation samples to prove the existence of producible hydrocarbons in the rock pores. In a down hole fluid sampling process, the primary objective is to obtain or identify formation samples representative of true, for example, clean formation fluid or native fluid with a low contamination level of borehole fluids or drilling fluids.
During the pumping process, physical and chemical properties of formation fluids being extracted from the formation can be measured using sensors placed along the flowline of the tool. The sensor measurements are used to try and identify the fluid type, and to calculate the contamination level.
The pumped fluids usually consist of a mixture that is segregated where a single measurement in time cannot be made that accurately represents the bulk fluid mixture, and the data measurements are erratic. While an average of the data can be made, the average of the data often fails to accurately identify the fluid type and levels of contamination.